Electrically conducting coating on glass and other ceramic bodies



2,564,677 BODIES 1947 J. K. DAVIS ELECTRICALLY CONDUCTING COATING ONGLASS AND OTHER CERAMIC Filed Sept. 15

Aug. 21, 1951 AIR lQ/D/ZED 0x105 COflT/NG GLASS PLATE I ilnbmtur JmmssA. 24 W5 Zltinmw lR/D/ZED G JUPE/PPOSED OXIDE C019 77 hereinafter bereferred Patented Aug. 21, 1951 ELECTRICALLY CONDUCTING COATING ON GLASSAND OTHER CERAMIC BODIES James K. Davis, Corning, N. Y., assignor toCorning Glass of New Yor I;Works, Corning, N

. Y., a corporation Application September 15, 1947, Serial No. 773,992

:6 Claims. (Cl. 201-73) This invention relates to electric heatingdevices and others comprising a base member of glass or other ceramicbody provided with a eating element comprising an electricallyconducting coating of the kind known generally as iridized coatings.When glass or other vitreous ceramic body is heated and contacted withcertain metal salts either in the form of fumes or atomized solutionsthereof, a strongly adherent layer comprising an oxide of the metal isformed on its surface. This process is known as iridizing, because thecoatings thus produced are frequently iridescent due to interference oflight waves reflected from the extremely thin films with those reflectedfrom the glass.

The application of iridizing to glass for the production of beautifulartware is quite old, and for this purpose salts of tin and of iron areemployed. Recently it has been found that iridized coatings comprisingcertain combinations of oxides of cadmium, indium, tin, and antimonyhave sufficiently low electrical resistivities to permit their use aselectrical conductors for electric heating devices, particularly whendeposited on a transparent glass supporting member, as described inapplication Serial No. 771,859, filed on September. 3, 1947, by John M.Mochel.

Insofar as it has been possible to determine by chemical and X-rayanalyses, the iridized coatings, which are mentioned above, are composedof metal oxides. Consequently, they will to as being composed of oxides,although the fact that some of the coatings are electrically conductingraises a doubt that such a definition is sufficiently exact. In moderntheory the electrical conductivity of oxides is ascribed tonon-uniformities in their lattice structure resulting from a departurefrom the stoichiometric proportions of their constituents. In otherwords, the constituents of an electrically conducting oxidetheoretically may be present in somewhat smaller or greater, and thusfar indeterminate, proportions than would correspond to itsstoichiometric composition. In

view of such uncertainty and in the absence of a more exact term bywhich to define the iridized coatings of this invention, they arehereinafter referred to as iridized or "oxide coatings, it beingunderstood that the future determination of a proper definition for suchcoatings shall not in any way change the scope of the claims hereuntoappended.

It has been found that when such a conducting coating is deposited onone side of a glass plate and an electric current is passed through thecoating for the development of heat, the amount of radiant heat which isemitted from the coated side of the plate, that is, the outer face ofthe coating is less than the amount which is emitted from the other oruncoated side of the glass plate. Although such difierence in emissionis advantageous for some purposes, as is pointed out in the Mochelapplication above referred to, it is desirable in some circumstancesthat the emission from both sides of the glass plate be equal or morenearly so.

The primary object of this invention is to increase the emission ofradiant heat from the outer face of an electrically conducting oxidecoating on a ceramic body when the coating is heated by an electriccurrent passed therethrough.

Another object is to equalize the emission of radiant heat from bothfaces of an electrically conducting oxide coating on glass when thecoating is heated by an electric current passed therethrough.

A further object is to protect an iridized oxide coating on a ceramicbase against atmospheric influences.

Still another object is to stabilize the electrical resistance of aconducting iridized oxide coating on a ceramic base.

Another object is to electrically insulate an electricallyconductingoxide coating on a ceramic base member without altering the electricalresistivity of theconducting coating.

To these and other ends the invention includes the article comprising aceramic supporting member provided with a conducting oxide coating and asuperposed oxide coating of different composition and higher electricalresistance coating, to be hereinafter more fully described andillustrated in the accompanying drawing in which:

Fig. 1 is an elevation of an apparatus for depositing oxide coatings ona glass sheet in accordance with the inventiony Fig. 2 is a view inperspective of a glass plate provided with oxide coatings in accordancewith my invention but partly broken away for convenience in viewing andgreatly exaggerated in thickness, and

Fig. 3 is a vertical diagrammatic section of an electrically heateddevice for toasting bread, comprising an electrically heated glasselement provided with iridized oxide coatings, greatly exaggerated, madein accordance with my invention.

I have found that if an electrically conducting iridized coatingsuitable for generating heat, that -ide coating of different compositionand higher resistance, there will be an increase in the amount of heatwhich is radiated from the outer face of the conducting coating when thelatter is heated by the passage of an electric current therethrough. Ihave also found that such a superposed iridized oxide coating has otherfunctions and advantages. For example, certain atmospheric influences,such as humidity, cause fluctuations in the resistance of a conductingiridized oxide coating which a superposed coating of higher resistanceappears to prevent. A superposed iridized coating may also function toinsulate a conducting undercoating or to shield the latter againstabrasion.

The iridized oxide films of this invention may be formed on the surfaceof a glass article such as a glass plate. Referring to Fig. 1, a glassplate is heated uniformly on an electric hot plate II. An atomizer,generally designated l2 and preferably composed of glass, comprises acup I3 for or mixture of salts to be hereinafter described. Thesuperposed coating may be deposited immediately after the formation ofthe first coating and before the glass has cooled, or may be depositedafter any desired interval of time, the glass being reheated and treatedas before.

As pointed out above iridized oxide coatings comprising certaincombinations of oxides of cadmium, indium, tin, and antimony areelectrically conducting. These metals have atomic weights ranging fromabout 112 to about 122 and are numbered from 48 to 51 respectively inthe periodic system of elements. For making conducting coatings, theypreferably are combined thus: tin oxide with antimony oxide, or tinoxide with indium oxide, or cadmium oxide with indium oxide. Other metaloxides, such as oxides of zinc. copper, iron, manganese, cobalt,vanadium, etc., may be added in small amounts, not exceeding say aboutto the conducting compositions of tin oxide and antimony oxide, or tinoxide and indium oxide, for the purpose of modifying their properties.Tin oxide alone is also effective in forming electrically conductingcoatings for purcontaining the solution of salts to be atomized, anposes within the scope of this invention. To a atomizing nozzle l4 and atube l5 for the introlesser extent cadmium oxide alone and indiumduction of compressed air to the nozzle. The oxide alone also areeffective, but antimony oxide atomizer is so supported that the nozzlei4 is alone formsanon-conducting coating. The comabout one foot abovethe glass plate. binations consisting of tin oxide and cadmium By meansof the hot plate H, the glass plate In oxide, or cadmium oxide andantimony oxide, or is heated to a temperature above 500 C.,Preferantimony oxide and indium oxide, form coatings ably to 650 C., andthe solution in the cup I3 is which have too high resistances forelectrical atomized as a fine mist on to the heated plate l0 heatingpurposes, b p s i i through e tube 15 for about 0 to The superposedcoatings of this invention 20 seconds. The metal salts are decompo d yshould have higher electrical resistances than hydrolysis and heat toform a thin strongly-adherent film of the corresponding metal oxides onthe surface of the glass.

The thickness of the oxide film may be gauged by the apparent color ofthe film caused by interference of light waves reflected therefrom. As

the under coatings. In some proportions the oxides of tin and antimonyform high resistance coatings which radiate heat so eificiently thatthey are particularly suitable for present purposes. Other metal oxidesper se may also be used to form high resistance superposed coatthethickness of the fllm increases, its apparent color changes and theorder or succession of the colors with increasing thickness is analogousto thatof the well-known Newton rings described in A Treatise on Light,by R. A. Houstoun, Longmans, Green 8: Co., Ltd., (1938) page 147. Sincered marks the end of each order, this color is preferably employed asthe distinguishing mark of the successive orders and. for presentpurposes, is assigned a wave length of 6200 angstroms. Calculation showsthat the approximate film thickness in angstroms for the various ordersof red is ings.

For the purpose of this invention, the use of coatings containing oxidesof tin and antimony is preferable both for undercoatings and forsuperposed coatings because, when the tin oxide content is high, theyproduce conducting coatings of lower electrical resistance and higherelectrical stability than other oxides and, when the antimony oxidecontent is high, they produce high resistance coatings havingparticularly high heat emissivity.

The following compositions are examples. but

approximately not limitations, of solutions which can b iused o d A toproduce conducting coatings and superposed ngstmms coatings forincreasing the heat emission of the 1 775 conducting coatings, and forother purposes in a 23g accordance with the invention: a aggg TABLE ICompositions for conducting coatings, expressed For convenience, theresistance in ohms of a in grams and cubic centimeters square film ofthird order red is herein arbitrarily employed as the unit of specificresistance or the 1 2 3 4 5 6 films herein described, and in order toavoid confusion is designated standard resistance or snC1.. m0 100 100100 100 100 S. R. The resistivity may be calculated by mul- 0625 1 2 4tiplying the "standard resistance by the thickv ness in centimeters of athird order red film gjig z' (3870 x 10- cm.) Incl,

For present purposes another coating of higher 83 1 8 3? .11 resistanceis deposited over the first coating, as ,6 53159;; "55 55"55"5555 "5'';{3"}; i8 Shown inFig. 2, by iridization as described above, H0100 0 1010 10 10 10 2 2 using a diiferent solution of a suitable metal saltTABLE II than the coating over which it is to be superposed. The use 01such superposed coatings also results mi iz $$$Z$Z$ifi in protection orthe conducting coating against atmospheric influences and causes animprove- 12 13 M 16 17 18 5 ment in its electrical stability. Othercompositions having the proper relative resistances may be used in lieuof those given above. St8ifi9113313::::::::::::: it 3. 11139.19. 1% Toillustrate electric heatin devices made in accordance with my invention,reference is had I: "5' III: III 10 to Fig. 3 which representsdiagrammatically an 8 366 electric bread toaster comprising threeparallel 20 so 10 10 10 50 glass plates 20, 2|, and 22, and two slicesof bread to be toasted 23 and 24 which are placed one on In lieu of thesalts recited in Tables I and II, h si f he t ally l a d plate and otherequivalent soluble salts may be employed, 15 parallel therewith. Theglass plates are provided for example, stannous chloride, antimonypentawith irldized electrically conducting oxide coatchloride, etc. Thepercentages of the correspondlugs 25, 26, and 21 respectively, and thecentrally ing oxides in the above solutions are as follows: locatedplate 2| is additionally provided with a TABLE III Conducting coatingsCdO S. R. Ohms 43 17 55 73 55 32 28 TABLE IV superposed oxide coating28, all of the coatings being greatly exaggerated in thickness for con-Superposed coatings venience in viewing. The glass plates 20, 2|, and 22are further provided along their upper edges with metal strips 29, 30,and 3| respectively, and along their lower edges with metal strips 32,33,

5 and 34 respectively. The metal strips 28 through 11.5 34 are integralwith the respective glass plates and are in electrical contact with theconducting 0:10,. u I: oxide coatings thereon. The glass plates 20, 2|,Per Cent Emissionm- 79 82 and 22 and the bread slices 23 and 24 aresupported in a vertical position on a dielectric base 35 The abovecompositi s and many othershaving clips as, :n, and as which are inelectrical which are too numerous to be here s fOI'th but contact withthe lower metal strips 32, 33, and

which are disclosed in the above-entitled applica- 34 respectively. Theclips 5 31 and 33 are election or known to the art, may be ap d to glasstrically connected with a wire 39, and the upper or other ceramicarticles by the above-described metal strips 29 3 and 3 are electricallymethod. When one of the above or other connected t a wire 40, The wires39 d 4 form ducting coatings is deposited upon a glass pla a circuitwith a source of electric current (not and heated to 350 C. by thepassage 0f sufficient shown) Any known device (not hown) for electriccurrent therethrough, the amount of heat removing t bread d interruptingt flow of which is radia ed f o e film e is 0011511191 current either bymanual or automatic means ably less than the amount which is radiatedfrom may be employed, the glass side. In some cases the emission ratio,w about 11 tt per Square inch t 110 or ratio of film radiati n to gl r imay volts is passed through the conducting coatings beless than .5 or50%. For examp e, afi m about 25, 26, and 21, the coatings are heated toabout 4000 angstroms thick of composition 6 has a 350 C. From the plates20 and 22 more radiant emission rat o of and e 1 Composition 2 50 heatis emitted from their uncoated faces than has an emission ratio of 35%.from their coated faces, and the uncoated face of Tho m on," recited inTable IV for comeach plate is therefore turned toward the adjacent Pitions 12 to 18 inclusive, is the emission ratio slice of bread. On theother hand, the centrally which results when coatings from 1000 to 4000h t d glass plate 2|, by virtue of the superposed angstroms thick ofthese compositions are sutin 23, mit substantially a m h adia t, n p s dn a -ti m d f m mp s ti heat from its coated face as from its uncoatedIt will be noted that the increase in heat emisface, and the lices ofbread 23 and 24 are subsion over that of composition 6 alone is quiteSubstantially equally affected thereby. stantial.

Other arrangements of the various parts of the n n l, the electricalresistance of comdevice and modifications thereof for other purpositions12 to 18 inclusive is higher than the reposes will be apparent to thoseskilled in the art sistance of compositions 1 to 11. In selecting a andare included within the scope of the invencomposition for superpositionover'a conducting tion as claimed.

coating for increasing the emission thereof, it is I claim:

necessary to use one having a higher resistance 76 1. An electricheating device which comprises a vitreous ceramic body having on asurface thereof an electrically conducting iridized metal oxide filmintegrally united with such surface and having an electrical resistanceof less than 1000 ohms per square and, superposed on said film andintegrally united therewith, a second iridized metal oxide film having adifferent composition and a higher electrical resistance than the firstfilm, said body being provided with spaced metallic members inelectrical contact with the film integrally united with the surface ofsuch body.

2. An electric heating device which comprises a vitreous ceramic bodyhaving on a surface thereof an electrically conducting iridized metaloxide film comprising an oxide of tin and an oxide of antimonyintegrally united with such surface and having an electrical resistanceof less than 1000 ohms per square and, superposed on said film andintegrally united therewith a second iridized metal oxide film having adifferent composition and a higher electrical resistance than the firstfilm, said body being provided with spaced metallic members inelectrical contact with the film integrally united with the surface ofsuch body.

3. An electric heating device which comprises a vitreous ceramic bodyhaving on a surface thereof an electrically conducting iridized metaloxide film comprising an oxide of indium andan oxide of tin integrallyunited with such surface and having an electrical resistance of lessthan 1000 ohms per square and, superposed on said film and integrallyunited therewith a second iridized metal oxide film having a differentcomposition and a higher electrical resistance than thefirst film, saidbody being provided with spaced metallic members in electrical contactwith the film integrally united with the surface of such body.

4. An electric heating device which comprises a vitreous ceramic bodyhaving on a surface thereof an electrically conducting iridized metaloxide film comprising an oxide of cadmium and an oxide of indiumintegrally united with such surface and having an electrical resistanceof less than 1000 ohms per square and, superposed on said film andintegrally united therewith a second iridized metal oxide film having adifferent composition and a higher electrical resistance than the firstfilm, said body being provided with spaced metallic members inelectrical contact with the film integrally united with the surface ofsuch body.

5. An electric heating device as claimed in claim 1, in which the filmintegrally united with the surface of the body consists of an oxide oftin. an oxide of antimony, and not over 20% ZnO.

6. An electric heating device as claimed in claim 1, in which the filmintegrally united with the surfac of the body consists of about 93%S1102, 5.5% SbzOs, and 1.5% ZnO, and the superposed film consists ofabout 76% SnOz, 12.5% SbzOa, and 11.5% ZnO.

'JAMES K. DAVIS.

REFERENCES CITED The following references are of record in the file ofthis'patent:

UNITED STATES PATENTS

1. AN ELECTRIC HEATING DEVICE WHICH COMPRISES A VITREOUS CERAMIC BODYHAVING ON A SURFACE THEREOF AN ELECTRICALLY CONDUCTING IRIDIZED METALOXIDE FILM INTEGRALLY UNITED WITH SUCH SURFACE AND HAVING AN ELECTRICALRESISTANCE OF LESS THAN 1000 OHMS PER SQUARE AND SUPERPOSED ON SAID FILMAND IN-